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<body class="article">
<div id="header">
<h1>CCNx Binary Encoding (ccnb)</h1>
</div>
<div id="content">
<div class="sect1">
<h2 id="_design_target">Design Target:</h2>
<div class="sectionbody">
<div class="ulist"><ul>
<li>
<p>
Custom binary encoding format for XML to meet specific needs of CCNx
</p>
</li>
<li>
<p>
Encoding that would be attractive for others to adopt for general use independent of CCNx
</p>
</li>
</ul></div>
</div>
</div>
<div class="sect1">
<h2 id="_goals">Goals:</h2>
<div class="sectionbody">
<div class="ulist"><ul>
<li>
<p>
binary to xml to binary: output should exactly match input
</p>
</li>
<li>
<p>
xml to binary to xml: output not guaranteed to exactly match input
</p>
</li>
<li>
<p>
support evolution while still making binary to xml to binary transformation possible
</p>
</li>
</ul></div>
</div>
</div>
<div class="sect1">
<h2 id="_format">Format:</h2>
<div class="sectionbody">
<div class="ulist"><ul>
<li>
<p>
transmission unit: octet, high-order transmitted first
</p>
</li>
<li>
<p>
encoding unit: "blocks": represents start of XML element or attribute, or character data
</p>
<div class="olist arabic"><ol class="arabic">
<li>
<p>
Header
</p>
<div class="olist loweralpha"><ol class="loweralpha">
<li>
<p>
number value (non-negative integer, variable-length big-endian representation, high order 1 is terminator, packed 7 bits per byte except for the last byte, which gets 4 bits)
</p>
</li>
<li>
<p>
3 bits type (low-order bits): (TT) determines interpretation of rest of "block"
</p>
</li>
</ol></div>
</li>
<li>
<p>
Value - presence and length depend on the block type TT
</p>
</li>
</ol></div>
</li>
<li>
<p>
Ending delimiter: represents close of XML element
</p>
<div class="olist arabic"><ol class="arabic">
<li>
<p>
single octet (0x00)
</p>
</li>
</ol></div>
</li>
</ul></div>
</div>
</div>
<div class="sect1">
<h2 id="_block_types">"Block" Types:</h2>
<div class="sectionbody">
<div class="dlist"><dl>
<dt class="hdlist1">
BLOB(TT=5)
</dt>
<dd>
<p>
     Arbitrary binary value, arbitrary length (including 0), no alignment requirements
</p>
<div class="ulist"><ul>
<li>
<p>
number value is length in octets
</p>
</li>
<li>
<p>
Value will encoded as base 64 when converted to text XML
</p>
</li>
</ul></div>
</dd>
<dt class="hdlist1">
UDATA(TT=6)
</dt>
<dd>
<p>
     same as BLOB but value restricted to valid UTF-8 sequences
</p>
</dd>
<dt class="hdlist1">
TAG(TT=1)
</dt>
<dd>
<p>
     XML element tag name in UTF-8 encoding
</p>
<div class="ulist"><ul>
<li>
<p>
number value is length in octets of UTF-8 encoding of tag name - 1 (minimum tag name length is 1)
</p>
</li>
<li>
<p>
may be followed by anything due to XML nesting
</p>
</li>
</ul></div>
</dd>
<dt class="hdlist1">
ATTR(TT=3)
</dt>
<dd>
<p>
     XML attribute name in UTF-8 encoding
</p>
<div class="ulist"><ul>
<li>
<p>
number value is length in octets of UTF-8 encoding of attribute name - 1 (minimum attribute name length is 1)
</p>
</li>
<li>
<p>
UDATA must follow immediately representing attribute value (though 0 length value may be represented)
</p>
</li>
</ul></div>
</dd>
<dt class="hdlist1">
DTAG(TT=2)
</dt>
<dd>
<p>
      Dictionary Tag
</p>
<div class="ulist"><ul>
<li>
<p>
Same as TAG except number value is an index into a dictionary of tag names (for compressed encoding of tag names)
</p>
</li>
<li>
<p>
Dictionary must be agreed separately or from context
</p>
</li>
</ul></div>
</dd>
<dt class="hdlist1">
DATTR(TT=4)
</dt>
<dd>
<p>
      same as DTag but for attribute names
</p>
<div class="ulist"><ul>
<li>
<p>
Distinct dictionary for tags and attributes
</p>
</li>
</ul></div>
</dd>
<dt class="hdlist1">
EXT(TT=0)
</dt>
<dd>
<p>
      Anything other than the XML structures covered by explicit types above, e.g. processing instructions.
</p>
<div class="ulist"><ul>
<li>
<p>
number value is extension subtype identifier (identifiers to be allocated as extensions are defined)
</p>
</li>
</ul></div>
</dd>
</dl></div>
</div>
</div>
<div class="sect1">
<h2 id="_environment">Environment</h2>
<div class="sectionbody">
<div class="ulist"><ul>
<li>
<p>
Dictionary: interpretation depends on agreed external dictionaries for tags and attributes that will be commonly used
</p>
<div class="ulist"><ul>
<li>
<p>
For CCNx, a <a href="DTAG.html">single, static dictionary</a> is used for the DTags
</p>
</li>
<li>
<p>
The CCNx protocols do not use any DAttr encoding, so the dictionary is effectively empty
</p>
</li>
</ul></div>
</li>
</ul></div>
</div>
</div>
<div class="sect1">
<h2 id="_text_conversions">Text Conversions</h2>
<div class="sectionbody">
<div class="ulist"><ul>
<li>
<p>
When decoding to standard XML text, the arbitrary octets of a BLOB value are encoded as a sequence of UTF-8 characters according to a standard encoding
</p>
<div class="ulist"><ul>
<li>
<p>
Base 64 is the default encoding (base64Binary)
</p>
<div class="olist arabic"><ol class="arabic">
<li>
<p>
Alternative encodings (e.g. hexBinary) may be used if called for by a schema governing the XML that is produced
</p>
</li>
</ol></div>
</li>
<li>
<p>
A special attribute value named <em>ccnbencoding</em> will be added to the tag of any element containing such BLOB values so that the XML text may be encoded to produce the original binary encoding - i.e. the UTF-8 encoding of the arbitrary value will be reversed when the special attribute value is specified on the containing element
</p>
</li>
<li>
<p>
The special attribute value may be specified implicitly by the governing schema definition rather than explicitly included in every tag where it applies
</p>
</li>
<li>
<p>
XML attributes may not have BLOB-encoded values so that no issue arises for converting them to text representation
</p>
</li>
</ul></div>
</li>
</ul></div>
</div>
</div>
</div>
<div id="footnotes"><hr /></div>
<div id="footer">
<div id="footer-text">
Last updated 2011-05-04 08:48:35 PDT
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